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White Smoke after shutdown

Hey guys! I've got a question for you about a clients furnace. I'm a licensed oil burner tech that works for a company that hires heating vendors to repair/replace heating systems for low income and elderly people. We helped replace an oil fired furnace about 3 years ago to a family and up until this past year it worked fine. Its an Olsen lowboy with a beckett burner. The client called us to complain that occasionally he was getting smoke/ fumes in the basement and the home. Someone told him the heat exchanger was cracked.

We ended up having his oil vendor check it out and he couldn't find a problem. About a week later we got another complaint from the HO about the same issue. This time we had the oil vendor, the company that installed the furnace, and the Olsen rep at the home....No problem was found. Third complaint about the same issue came in. We had a different vendor check it out and he replaced the pump (because he said the solenoid was intermittently opening and closing), fuel filter, oil line (he said pin hole leak), added a check valve, and found a draft issue (he made repairs to the chimney).

I just got involved in this case recently and had to inspect it to verify the parts were installed. I went there and fired the system. I did a combustion test with a digital analyzer before the fan came on and after the fan came on to verify there was no heat exchanger leakage (which there wasn't). I shut the switch off to remove my jumpers, turn the switch back on (the fan came back on) and all of a sudden white smoke starts billowing out from the burner! I was glad it did it when I was there because no one else has actually been there when its happened.

My question is since they already replaced the pump do you think the drawer assembly could have a crack in it? I would think a simple cut off test would show there is a leak somewhere after the pump right? If you think I am wrong I would love some of the experts in here to give me there opinion. I really need this thing fixed before the next season. Thanks

Even if there is after drip the smoke shouldn't be coming out the front of the burner, it should be going up the flue. So you either have some combustion air issues or possibly a wet chimney.

An oil burner will suck all the combustion air it needs when running, however if there are negative pressure problems that will cease when the burner shuts down. The key is the draft reading, above the baromtric, when the burner shuts down. If the draft disappears within seconds after the burner shuts down the above is your problem.

This is where monitoring CO not CO2 during the Light-off(spike) Run and Shutdown(spike) can be very helpful. The smoke may just be what was in the flue when it shut down and it came back in. The smoke is white because it is mostly clean and contains water.

Combustion air problems are sometimes as easy as adding a supply register to the plenum. A wet flue problem usually means there is no flue cap or the furnace is underfiring and condensing.

If there was a hole in the heat exchanger the smoke would be coming out the whole time not just at the end.. CO readings, Flue T and draft readings can easily solve this problem versus just guessing which has already been done.

If I was guessing I would say the furnace is sending out smoke signals for help!!